Stereoselective synthesis of β-d-GlcNAc-(1 → 4)-d-Glc disaccharide starting from lactose ☆

Abstract The stereoselective preparation of the β- d -GlcNAc-(1 → 4)- d -Glc disaccharide starting from known 4- O- [6- O -(1-methoxy-1-methylethyl)-3,4- O -isopropylidene-β- d -talopyranosyl]-2,3:5,6-di- O -isopropylidene -aldehydo - d -glucose dimethyl acetal ( 2 ), in turn easily obtained from lactose, is reported. Key steps of this new procedure, that avoids the glycosylation reaction, are (a) a first epimerization at C-4′ through an unusual procedure involving a completely stereospecific hydroboration–oxidation of the enol ether group of the hex-4-enopyranoside 4 , obtained from 3 by base promoted acetone elimination, (b) an amination with inversion by S N 2 reaction on an imidazylate intermediate, and, finally, (c) N-acetylation followed by complete deprotection.